This invention relates to a mineral-filled rubber composition derived in part from sulfur-containing silane coupling agents and articles made from the rubber composition such as tires, tire components, power transmission belts, etc.
Known sulfur-containing silane coupling agents employed in the production of mineral-filled rubbers (elastomers) contain one or more of the following chemical bond types: SH (mercapto), S—S (disulfide or polysulfide), C═S (thiocarbonyl) and C(═O)S (thioester).
Mercaptosilanes exhibit high chemical reactivity for known and conventional organic polymers used in mineral-filled elastomers and therefore effect coupling at substantially reduced loadings. However, the chemical bonds between mercaptosilane coupling agents and organic polymers tend to be weaker than the carbon-carbon bonds of the organic polymers. Under high stress and/or high frequency use conditions, these chemical bonds are susceptible to breakage and, therefore, loss or reduction of coupling between the organic polymer and the coupling agent. This loss or reduction of coupling may contribute to the wear and/or degradation of other important physical properties of rubber compositions formulated with mercaptosilane coupling agents. The high chemical reactivity of mercaptosilane coupling agents with organic polymers also leads to unacceptably high viscosities'during processing and/or premature curing (scorch). Their undesirability is further aggravated by their highly disagreeable odor. As a result, other less reactive coupling agents such as those containing the S—S (disulfide or polysulfide) linkage and C═S (thiocarbonyl) or C(═O)S (thioester) functional groups are used alone or in admixture with mercaptosilane coupling agents in order to reduce the high chemical reactivity of the latter. Because these less reactive silane coupling agents generally contain a single sulfur functional group capable of reacting with the organic polymers, the relatively weak C—S bond may break when the rubber is subjected to high stress use conditions leading to decoupling of the rubber from the mineral filler, formation of voids and/or tearing of the filled rubber which manifests itself most commonly as wear.
M. G. Voronkov et al., Zhumal Obshchei Khimii (1975), 45(6), 1395, discloses acylthioalkyl silanes such as CH3C(═O)S(CH2)1-3Si(OR)3. U.S. Pat. No. 3,922,436 discloses the sulfur silane HOC(═O)CH2CH2C(═O)S(CH2)3Si(OC2H5)3 and its use as a coupling agent. This silane contains only a single blocked mercapto group.
JP 63270751 discloses the use of compounds represented by the general formula CH2═C(CH3)C(═O)S(CH2)1-6Si(OCH3)3 in tire tread compositions. These compounds contain only a single blocked mercapto group and are generally undesirable due to the potential of the unsaturation α,β to the carbonyl group of the thioester to undergo polymerization during the compounding process or during storage.
Australian Patent AU-A-10082/97 discloses the use in rubber of silanes of the structure R1nX3-nSi-(Alk)m(Ar)p—S(C═O)—R where R1 is phenyl or alkyl; X is halogen, alkoxy, cycloalkoxy, acyloxy or OH; Alk is alkyl; Ar is aryl; R is alkyl, alkenyl or aryl; n is 0 to 2; and, m and p are each 0 or 1 but both cannot be zero. However, it is stipulated that compositions containing these silanes must be used in conjunction with functionalized siloxanes.
U.S. Pat. Nos. 6,608,125; 6,683,135; 6,204,39; 6,127,468; 6,777,569; 6,528,673; and, 6,649,684, U.S. published patent application Nos. 2005/0009955, 2004/0220307, 2003/020900 and 2003/0130388, pending U.S. patent application Ser. Nos. 11/105,916 and 10/128,804 and EP 1 270 657 disclose the use of blocked mercaptosilanes of the structure [[(ROC(═O))p-(G)j]k—Y—S]r-G-(SiX3)s where Y is a polyvalent blocking group, r is an integer of from 1 to 3 and s is preferably from 1 to 3, in rubber master batches and as a surface treatment for mineral fillers. Although these patents and patent applications disclose coupling agents, that possess more than one blocked mercapto group, i.e., compounds in which r=2 or 3, they do not describe the specific stereochemical configurations of the polyvalent G structure between the silicon atom and the organofunctional group necessary to achieve efficient multiple bonding with the organic polymer component(s) of the rubber formulations.
U.S. Pat. Nos. 6,359,046; 5,663,226; 5,780,531; 5,827,912; 5,977,225; 4,709,065 and 6,759,545 and WO 2004000930 disclose a class of polysulfide silane coupling agents that on average contain more than one S—S (disulfide or polysulfide) linkage per molecule. However, the multiple S—S linkages are achieved by separating individual linkages with an organic hydrocarbon radical. In use, these S—S groups decompose to form sulfur radicals that couple to the polymer but generate species that contain only one sulfur reactive group per silicon atom. The polysulfide silane coupling agents disclosed in U.S. Pat. Nos. 5,110,969, 6,268,421, 6,211,345 and 6,350,797 overcome this drawback by containing more than one sulfur functional group directly bonded to a silicon atom through a cyclic hydrocarbon radical. The multiple S—S groups are bonded to adjacent carbon atoms and the silicon atoms are directly bonded to the ring structures through hydrosilation of the alkoxysilane with vinyl-containing cyclic hydrocarbons. However, these compounds contain rings of S—S and carbon atoms or are polymeric materials wherein the silyl-containing hydrocarbon radicals are bonded through S—S groups. These cyclic or polymeric coupling agents are less reactive for organic polymers because they contain S—S groups bonded directly to secondary carbon atoms. The bonding of the S—S-containing group to secondary carbon atoms sterically hinders the reaction of the S—S groups and inhibits their reaction with the organic polymers.
U.S. published patent application No. 2008/0161590 discloses sulfur silanes containing multiple blocked mercapto groups and the use of the silanes in rubber. These sulfur silanes form multiple attachments to the rubber and effectively couple the rubber to the mineral filler. However, due to the relatively large bulky cyclo group, the filler can possess regions that do not contain silane coupling agent.
Therefore, a need exists for sulfur-containing silane coupling agents that have low reactivity to provide processing of the mineral-filled elastomers or rubbers but without scorch, that aid in the effective dispersion of the mineral filler within the rubber matrix and that can be activated at the desired time to form multiple linkages with the organic polymer component(s). These multiple linkages provide sufficient bonding so that the loss of coupling between the rubber and coupling agent is minimized during high stress use conditions, such as those experienced by tires, without exhibiting the disadvantages described above.